Phase shift keying (PSK) is a method, for taking data on a high frequency of carrier wave and sending it. PSK transmits a signal in which converting any one of a phase, an amplitude and a frequency of the carrier wave or a combination, thereof, in converted into digital data of 0 and 1.
In the PSK method, there is known a binary phase shift keying (BPSK) method, which transmits digital signals of two values (0 and 1) while making them correspond to 0 phase and π phase of the carrier wave, respectively. There is also known a quadrature phase shift keying (QPSK) method, which gathers and transmits two bits of 0 and 1, which are digital signals of two values, while making them correspond to four phases of the carrier wave.
Also, there are known an 8 PSK method, which can transmit information three times larger than in the BPSK method, and a 16 PSK method, which can transmit information four times larger than in the BPSK method, at the same frequency band as that of the BPSK method.
A method of recovering data from a received signal is known, which adjusts a sine wave of a receiving end to have the same phase as a carrier wave of a transmitting end by using a feedback loop and then multiplies the received signal by the adjusted sine wave to recover the data.
An apparatus, which is mostly used in such a method, is a COSTAS-loop. A construction of a related art COSTAS-loop as illustrated in FIG. 1. The COSTAS-loop illustrated in FIG. 1 is made up of first, second, and third mixers 110, 160, and 130, first and second low pass filters (LPF), a loop filter 140, and a voltage controlled oscillator (VCO) 150.
The first mixer 110 mixes a modulated signal m(t)cos(ωt) inputted from the outside and a sine wave signal cos(ωt+θ), which is an oscillation signal outputted from the VCO 150, and supplies the mixed signal to the first low pass filter 120. The second mixer 160 mixes the modulated signal m(t)cos(ωt) inputted from the outside and a sine wave signal sin(ωt+θ), which is an oscillation signal outputted from the VCO 150, and supplies the mixed signal to the second low pass filter 170. Thus, if a receiving end has a phase difference as much as θ to a carrier wave of a transmitting end, the signals outputted from the first and the second mixers 110 and 160 are as follows, respectively.m(t)cos(ωt)cos(ωt+θ)=m(t){cos θ+cos(2ωt+θ)}/2m(t)cos(ωt)sin(ωt+θ)=m(t){sin θ+sin(2ωt+θ)}/2
The first and the second low pass filters 120 and 170 pass only required signals of low frequency band with filtering jitter signals of high frequency band from the inputted signals. The signals outputted from the first and the second low pass filters 120 and 170 are m(t)cos θ and m(t)sin θ, respectively. Since θ converges to 0, m(t) can be recovered when a phase difference comes to 0.
The third mixer 130 mixes the signals outputted from the first and the second low pass filters 120 and 170 and supplies the mixed signal to the loop filter 140. The mixed signal is supplied to the VCO 150 via the loop filter 140. The VCO 150 produces oscillation signals according to a voltage control on basis of the signal outputted from the loop filter 140, and supplies the oscillation signals, which have a phase difference of 90° to each other, to the first and the second mixers 110 and 120, respectively.
However, to recover a carrier wave of high frequency, when the COSTAS-loop as described above is used, there is a difficulty in embodying the LPF. Particularly, in embodying the LPF, which is an analog filter, even though a RC filter having simplified structure is used, a flatness of frequency response at a high frequency is not good. Also, since capacitors occupy a large area, a problem occurs, in that it is difficult to fabricate a small filter.